What combos do you have in your community tank(s) ?....Do you have any that you thought might get along and didn't....or something that shouldn't get along..but does??
I have some full grown Raphael's that I was leery of putting in a 110gal with BN's and juvie cory's...but they're doing fine and coexist with no problems.
I put a whole ton of Endlers in my 100 gallon Discus tank. At first the Discus went after them. I'm sure I lost a few. Now the Discus ignore them unless one strays from the group. A lone Endler is a target. Mostly the Endlers stay together (safety in numbers) and stay close to the top. Discus aren't the best hunters and Endler's are fast movers.
I tried a smaller group in an 100 gallon mostly Angel tank. I figured they wouldn't last long. They didn't.
All my fish get along fine. I have guppies and mystery snails together. then I have zebra danips and black neons, cardinals, and black skirts. Right now all the school fish are in the small tank with the boy guppies and they are being nice. When I move them back to the 55 they will have some BN friends (hopefully from Josh). :) Some combos I thought might not work out but did were putting Miss Betta in with the boy guppies. but she was nice and didn't bother even my longest delta finned guys. In fact, the boy gups are almost my most aggressive fish! Now that I'm not breeding and have them separated the crazy boys have sort of become gay and chase and court/harass each other just like they would the females. Some bad combos are putting all boy guppies together (mine and then some mystery fish someone dumped on me): the boys all shredded each others tails. I was pretty shocked. Also, I thought it would work to separate out all my fry into the shrimp tank; but the crazy ghost shrimp eagerly ate all the free food falling from the sky. :o
The ones I have that should not get along are my Gourami. Pearls and Gold. They were some of my fry. Raised up together and have never had a problem.
I think that a lot of the mixing problem can be solved/prevented, if you raise fish together.
I have 6 rummy nose tetras, 4 rainbows, 2 are turquoise and 2 boseman, 5 roseline sharks, 2 clown loaches, 2 peper corys and 2 albino corys, a bristlenose pleco, one angel fish, and a discus in my 72 gallon. I've had people tell me my roselines will eat my rummy's and they all school together even though roselines are huge compared to rummy tetras. I've been told I have to have 6 clown loaches and my tank isn't big enough for them and that part may be true because they can get 10-12 inches I think. Anyway I've been told several times the tankmates I have won't work. Right now they are all happy together. I really enjoy the variety.